target/riscv: convert abstract CPU classes to RISCVCPUDef

target/riscv: convert abstract CPU classes to RISCVCPUDef

Start from the top of the hierarchy: dynamic and vendor CPUs are just
markers, whereas bare CPUs can have their instance_init function
replaced by RISCVCPUDef.

The only difference is that the maximum supported SATP mode has to
be specified separately for 32-bit and 64-bit modes.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>